1. Field of the Invention
The present invention relates generally to memory, and more specifically to memory employing a cross point array.
2. Description of the Related Art
Conventional nonvolatile memory requires three terminal MOSFET-based devices. The layout of such devices are not ideal, usually requiring feature sizes of 8 f2 for each memory cell, where f is the minimum feature size.
However, not all memory elements require three terminals. Certain complex metal oxides (CMOs), for example, can retain a resistive state after being exposed to an electronic pulse, which can be generated from two terminals. U.S. Pat. No. 6,204,139, issued Mar. 20, 2001 to Liu et al., incorporated herein by reference for all purposes, describes some perovskite materials that exhibit such characteristics. The perovskite materials are also described by the same researchers in xe2x80x9cElectric- pulse-induced reversible resistance change effect in magnetoresistive films,xe2x80x9d Applied Physics Letters, Vol. 76, No. 19, 8 May 2000, and xe2x80x9cA New Concept for Non-Volatile Memory: The Electric-Pulse Induced Resistive Change Effect in Colossal Magnetoresistive Thin Films,xe2x80x9d in materials for the 2001 Non-Volatile Memory Technology Symposium, all of which are hereby incorporated by reference for all purposes.
Similarly, the IBM Zurich Research Center has also published three technical papers that also discuss the use of metal oxide material for memory applications: xe2x80x9cReproducible switching effect in thin oxide films for memory applications,xe2x80x9d Applied Physics Letters, Vol. 77, No. 1, 3 Jul. 2000, xe2x80x9cCurrent-driven insulator-conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals,xe2x80x9d Applied Physics Letters, Vol. 78, No. 23, 4 Jun. 2001, and xe2x80x9cElectric current distribution across a metal-insulator-metal structure during bistable switching,xe2x80x9d Journal of Applied Physics, Vol. 90, No. 6, 15 Sep. 2001, all of which are hereby incorporated by reference for all purposes.
Similarly, magnetic RAM (MRAM requires only two terminals to deliver a magnetic field to the memory element. Other two terminal devices include Ovonic Unified Memory (OUM), which uses chalcogenic layers of material, and various types of ferroelectric memory. With only two terminals, it has been theorized that memory can be arranged in a cross point architecture.
However, mere recognition that a two terminal memory element is theoretically capable of being placed in a cross point array does not solve many of the non-trivial problems associated with actually creating such a device.
The present invention provides a cross point memory array. In one embodiment, the memory array includes a first group of substantially parallel conductive array lines, a second group of substantially parallel conductive array lines and a plurality of memory plugs.
The second group of substantially parallel conductive array lines are oriented to be substantially perpendicular to the first group of parallel conductive lines and the plurality of memory plugs are located at the intersections of the first group of parallel conductive array lines and the second group of parallel conductive array lines.
Each memory plug includes multiple layers of thin films, including a thin film layer of a memory element and at least one thin film layer constituting a non-ohmic device. Thin films, as distinguished from thick films, are typically thin layers of material. Thick films are generally of a thickness greater than 0.1 mm while thin films are typically of a thickness less than 10 microns. The memory element switches from a first resistance state to a second resistance state upon application of a first write voltage pulse and reversibly switches from the second resistance state back to the first resistance state upon application of a second write voltage pulse, the second write voltage pulse having opposite polarity of the first write voltage pulse. The non-ohmic device imparts a relatively high resistance to the memory plug upon application of low magnitude voltages. The non-ohmic device imparts a relatively low resistance to the memory plug upon application of higher magnitude voltages, such as the first or second write voltage.